Ultrafast Demagnetization Excited by Extreme Ultraviolet Light From a Free-Electron Laser

Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-electron lasers (FELs) enable studying non-linear light−matter interactions on femtosecond timescales. Here, we report on the non-linear fluence dependence of magnetic scattering of Co/Pt multilayers, using FERMI FEL’s 70-fs-long single and double XUV pulses, the latter with a temporal separation of 200 fs, with a photon energy slightly detuned to the Co M2,3 absorption edge. We observe a quenching in magnetic scattering that sets-in already in the non-destructive fluence regime of a few mJ/cm² typically used for FEL-probe experiments on magnetic materials. Calculations of the transient electronic structure in tandem with a phenomenological modeling of the experimental data by means of ultrafast demagnetization unambiguously show that XUV-radiation-induced demagnetization is the dominant mechanism for the quenching in the investigated fluence regime of <50 mJ/cm², while light-induced changes of the electronic core levels are predicted to additionally occur at higher fluences. The modeling of the data further indicates that the demagnetization proceeds on the sub-20-fs timescale. This ultrashort timescale is consistent with non-coherent models for ultrafast demagnetization, considering the sub-femtosecond lifetime of hot electrons with energies of a few 10 eV generated by the XUV radiation.

Comparison of transient absorption of laser ablation plasma with fundamental plasma absorption relations

Nanosecond pulsed laser ablation plasmas were studied by time resolved shadowgraphy coupled with normal imaging, followed by laser probing and plasma spectroscopy in the 5-25 J/cm2 fluence regime. We describe methods for imaging and probing that allow us to determine variations in the distribution of ejecta in the plume and monitor the optical absorption using a probe laser to obtain a measure of the linear absorption coefficient of the plasma. Experimental determination of absorber distribution also corresponds well to the theoretical prediction of density increase near the emitted shockwave edge. We finally demonstrate that fundamental plasma correlations can accurately describe the absorption of light by the plasma near the ablation wavelength. We observed good agreement in peak attenuation, directly measuring 65% peak absorption and compared to a calculation of 57% using a simple model of the plasma, but a 10 ns shift in peak attenuation time. The shift in dip times is explained both by experimental error and a fundamental imprecision in the model proposed for the expansion.

Laser-induced incandescence (2λ and 2C) for estimating absorption efficiency of differently matured soot

Two-wavelength and two-color laser-induced incandescence (2λ–2C-LII) was used to study the absorption properties of three types of cold soot of different maturity from a mini-CAST soot generator. LII fluence curve analysis allowed for estimating absorption wavelength dependence in terms of dispersion coefficients ξ by the use of two excitation wavelengths (532 and 1064 nm). The estimated ξ (based on E(m, λ) ∝ λ1−ξ) spanned from ~ 1.2 for the mature soot, up to 2.3 for the young soot. The results for the mature soot showed good agreement with previous measurement using multi-wavelength extinction. For the young soot, however, some discrepancy was observed suggesting a weaker wavelength dependence (lower ξ) from the LII fluence analysis. Furthermore, an estimation of the E(m, λ) for the different types of soot was done from the experimental fluence curves with temperature analysis in the low-fluence regime and simulations using an LII model. Additionally, uncertainties and limitations were discussed. Finally, it should be pointed out that caution has to be taken when interpreting 2λ-LII results to obtain quantitative absorption properties of less mature soot, which may be influenced by thermal annealing during the laser pulse and by absorption from non-refractory species externally/internally mixed with the soot.

Depth Profiling of Polymer Composites by Ultrafast Laser Ablation

Past work has shown femtosecond laser ablation to be a non-thermal process at low fluences in polymer systems. The ablation rate in this low fluence regime is very low, allowing for micro-scale removal of material. We have taken advantage of this fact to perform shallow depth profiling ablation on carbon fiber reinforced polymer (CFRP) composites. Neat resin and composite samples were studied to establish reference ablation profiles. These profiles and the effects of the heterogeneous distribution of carbon fibers were observed through optical and scanning electron microscopy. Weathered materials that have been subjected to accelerated tests in artificial sunlight or high temperature conditions were ablated to evaluate any correlation between exposure and change in ablation characteristics. Preliminary Raman and micro-ATR analysis performed before and after ablation shows no chemical changes indicative of thermal effects. The low-volume-ablation property was utilized in an attempt to expose the sizing-matrix interphase for analysis.

140 MeV Si-ION IRRADIATION INDUCED Tc ENHANCEMENT IN YBCO THIN FILMS

On-line resistance measurement in a temperature range of 78 K to 124 K was performed on epitaxial thin films of YBCO irradiated with 140 MeV Si ions. The superconducting transition temperature (T c ), resistivity at 100 K (ρ 100 k ) and transition width (ΔT c ) all show peak at a fluence of 1.5×1013 ions/cm2. A crystallochemical analysis based on metastability induced charge and spin fluctuation due to irradiation induced oxygen disorder in the CuO chains explains the T c enhancement in the low fluence regime.

Time-Resolved Crystallization of GeTe

Amorphous thin films of GeTe were irradiated with excimer laser pulses and the subsequent crystallization was investigated utilizing simultaneous transient reflectivity and conductivity measurements. Below a threshold fluence of 15 mJ/cm2 pure thermal behaviour was found. Above that value, nucleation and growth are observed during the cooldown process. Above a fluence of 22 mJ/cm2 the films crystallize to a large degree within 200ns. Between 15 and 22 mJ/cm2 crystallite nuclei are formed (“frustrated crystallization”), and application of a subsequent pulse over areas exposed to this fluence regime leads to extremely fast crystallization (50 ns).